Excess argon in Mount St. Helens plagioclase as a recorder of magmatic processes

Details Excess argon in Mount St. Helens plagioclase as a recorder of magmatic processes Excess argon in Mount St. Helens plagioclase as a recorder of magmatic processes

Nov 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001georl..28.4279l&link_type=abstract

Geophysical Research Letters, Volume 28, Issue 22, p. 4279-4282

Physics

1

Geochemistry: Geochemical Cycles, Geochemistry: Geochronology, Geochemistry: Isotopic Composition/Chemistry, Volcanology: Physics And Chemistry Of Magma Bodies

Scientific paper

Excess argon in plagioclase crystals from young (<4000 ybp) tephra layers from Mount St. Helens, Washington, illustrates the importance of argon isotopes in understanding magmatic processes. 40Ar/39Ar step-heating identifies two distinct argon reservoirs in the plagioclase; a high Cl/K, low Ca/K reservoir with atmospheric 40Ar/36Ar and a low Cl/K, higher Ca/K reservoir with variable 40Ar/36Ar. The first is probably glass, whereas the second is ``true'' plagioclase. Felsic dacite plagioclases have little or no excess argon, but those from more mafic layers have significantly higher 40Ar/36Ar, indicating a non-atmospheric source. These variations are seen within a layer that contains both mafic and felsic dacite, and are inconsistent with either xenocrystic or restitic origin for plagioclase. The magma chamber exhibits long-term open-system degassing behavior punctuated by short-term fluctuations from influx of high 40Ar/36Ar basalt. The relative timing between mixing and eruption can affect the amount of excess argon recorded in plagioclase.

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